A non-aqueous electrolyte secondary battery, which has features of a higher operating voltage and higher energy density than that of a conventional nickel cadmium secondary battery or the like, has been widely used as a power source of an electronic device. Lithium transition metal composite oxides represented by LiCoO2, LiNiO2 and LiMn2O4 or the like are used as a positive electrode active material of the non-aqueous electrolyte secondary battery.
Among these, LiMn2O4 and LiMn2O4 where Mn is partially substituted with another metal (hereinafter, may be collectively referred to as lithium manganate) have the following advantages: a raw material thereof is inexpensively and easily obtained since manganese which is a construction element is present in great quantity as a resource; and the raw material has a low load to the environment. In order to utilize the advantages, a nonaqueous electrolyte secondary battery using the lithium manganate has been conventionally used in the application of a mobile electronic device typified by a portable phone, a laptop computer, a digital camera and the like.
In recent years, in the mobile electronic device, the demand characteristics have been further increased, due to the function advancement such as the application of various functions, and use at high temperature and low temperature, or the like. The non-aqueous electrolyte secondary battery is expected to be applied to power supplies such as batteries for electric automobiles, and the battery which can follow an abrupt start and abrupt acceleration of automobiles and enables high output high-rate discharge is desired.
Therefore, there is a need for the development of lithium manganate capable of smoothly inserting and desorbing Li. A positive electrode active material having micronized primary particles made of lithium manganate is considered as one particle design for embodying the development.
For example, as described in Patent Document 1, as the positive electrode active material obtained by micronizing the primary particle made of the lithium manganate, exemplified is a positive electrode active material obtained by pulverizing lithium manganate or the like using a dry type bead mill or the like.
Patent Document 2 proposes spherical hollow particles constituted by secondary particles having a spherical surface where primary particles are gathered by techniques such as a spray thermal decomposition method. As the spray thermal decomposition method, disclosed is a method including spraying a solution (not a slurry) containing lithium nitrate and manganese nitrate into a vertical type thermal decomposition furnace to thermally decompose the solution to obtain composite oxide powders, and then annealing the powders at a predetermined temperature.
Furthermore, Patent Document 3 proposes a high-density active material obtained by mixing a spherical manganese compound with a lithium compound while maintaining the shape of the manganese compound, and heat-treating the mixture. The active material has spherical particles filled with an active material substance.
Patent Document 4 discloses a positive electrode active material obtained by a production process of an active material for a nonaqueous electrolyte secondary battery mainly containing lithium and a transition metal. The positive electrode active material is an active material for a nonaqueous electrolyte secondary battery obtained by a process including pulverizing a lithium compound and a transition metal compound in a liquid medium to set the average particle diameter of the pulverized solid compound to 2 μm or less, then spray-drying the obtained solid-liquid mixture using a spray dryer equipped with a rotary disc type atomizer or a nozzle atomizer, and further burning a powder solid obtained by drying in the presence of oxygen. Patent Document 4 describes a lithium compound which is soluble in water as a liquid medium, as a preferred example, and a process for burning a powder solid obtained by spray drying as it is, as a burning step.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-48719    Patent Document 2: Japanese Patent Application Laid-Open No. 10-83816    Patent Document 3: Japanese Patent Application Laid-Open No. 2002-151079    Patent Document 4: Japanese Patent Application Laid-Open No. 10-106562